These active implantable medical devices (hereinafter referred to as “devices” or “implantable devices”) are generally in the form of a case containing a power supply (e.g., a battery) and a hybrid circuit board (hereinafter referred to as “electronic circuit”) supporting and interconnecting the various active and passive components, allowing for collecting and analyzing signals, generating electrical pulses, storing data such as medical follow-up information in memory, controlling the device's various functions, etc.
Manufacturing the device usually comprises a series of steps including, first, interconnecting the various functional elements, which are: the battery, the electronic circuit(s) as well as a series of feedthrough terminals intended to be subsequently connected to a corresponding terminal of a connector head. The following step concerns placing the resulting interconnected set of elements inside the case and then adding a connector head. The connector head will be used to ensure the mechanical and electrical connection of the case to external elements, preferably leads for collecting signals and/or delivering pulses.
The interconnection of the foregoing and similar functional elements is usually realized by a flex circuit comprising conductive tracks that are electrically and mechanically linked to the functional elements prior to integrating the functional elements within the case.
U.S. Pat. No. 6,245,092 proposes a cardiac pacemaker made using the flex circuit technology. The conductive tracks are present in the form of printed conductors that have a pad at their extremity which is intended to be linked through a metallization with a corresponding functional element. Brazing, the metallization technique used for the linkage, is a well-known technique providing excellent results in terms of electrical and mechanical performance and long-term reliability.
The use of brazing, typically Tin-lead (Sn—Pb) brazing, however, requires applying a brazing activation flux, followed by a careful cleaning of the substrate so as to remove any remainder of the flux. Until now, chlorofluorocarbon (CFC) based products could very efficiently clean flux from substrates. However, the prohibition against using CFC based products renders this cleaning step more difficult and creates an increased risk of forming metal dendrites resulting from the migration of Sn—Pb alloy to some areas of the substrates that have not have been prepared perfectly.
Moreover, future prohibitions against using brazing allows containing lead exacerbates these difficulties, specifically in the case of AIMD. Indeed, due to the limited room available within the case, the advanced miniaturization of these devices requires creating link pads with a very low pitch (typically 1.47 mm) and very narrow spacing between these pads (the interval between adjacent pads is typically about 250 μm). These problems, resulting from the extreme compactness of circuit area, is further amplified by the reduction of the size of the cases containing these circuits, whose volume has been recently reduced down to 8.5 cc.
U.S. Pat. No. 6,245,092, cited above, incidentally suggests avoiding brazing with lead, and replacing the brazing step with a gluing step. Gluing—which is a well-known and mastered technique in itself—for establishing electrical connections to the components, however, presents some major drawbacks. This technique essentially replaces brazing with a drop of conductive glue (silver glue), deposited on each of the contact pads of the flex printed circuit. This technique therefore requires that an individual deposit one drop onto each contact pad, and is therefore not appropriate for collective processing i.e., automated mass production, as contrasted with brazing (wave brazing for instance), which makes the whole process more complex and expensive.
Further, to avoid the risks of short-circuits between two adjacent pads, particularly if the pads are close to each other, it is highly recommended to deposit an insulating drop into the space separating the two conductive drops deposited on the adjacent pads. This gluing process becomes even more difficult as the circuit dimensions are reduced (pitch and interval between pads), which renders it inappropriate for such applications where advanced miniaturization is an essential requirement. Also, if the glue is in discrete drops, there is always a remaining risk that the silver particles that are in suspension within the conductive drops will migrate to the intermediate non-conductive drop.